You are talking about the "life table" used in the formulation of insurance rates. The dividend level in that year depends on the investment income of the insurance company in the previous year. Regardless of the income level, 70% of the income will be returned to the customer, and 30% of the insurance company will keep it as a reserve.
Further reading: How to buy insurance, which is good, and teach you how to avoid these "pits" of insurance.